The above requirements are fulfilled by the EUV plasma discharge lamp disclosed in WO 2005/025280 A2. The EUV lamp of this document comprises two electrode wheels arranged in a discharge space at a distance from one another to form a gap which allows the ignition of a plasma in a gaseous medium between the electrodes, as can be seen in FIG. 1. The electrode wheels 1 are rotatably mounted and partially dip into temperature controlled baths 2 comprising a liquid metal, for example tin. The material of the electrode wheels 1 allows the wetting of the electrodes by liquid tin, i.e. the surface of the electrode wheels 1 is covered with a thin layer of tin when rotating around rotation axis 3 through the tin baths 2. With a pulsed laser 4, tin is evaporated from one of the electrode wheels in the gap. The vapor cloud expands towards the second electrode wheel and after a certain time a short circuit is created between the electrode wheels. The capacitor bank 5, which is connected through an isolated feed through 6 to the tin baths 2, and therefore also to the electrode wheels 1, discharges and a hot plasma is created which emits the desired EUV radiation. The whole arrangement is situated in a vacuum vessel 8, which reaches at least a basic vacuum of 10−4 hPa. With this vacuum higher voltages from the capacitor bank 5 can be applied to the electrodes 1, for example 2 to 10 kV, without leading to an uncontrolled disruptive discharge. The tin layer 7 on the surface of the electrode wheels 2 is controlled in thickness by wipers 9. The thickness is typically controlled to be in the range between 0.5 μm and 40 μm. In order to avoid transport of evaporated tin to other parts of the lamp, metal shields 10 are arranged inside the lamp. Optical elements like mirrors outside the lamp are protected by a debris mitigation unit 11 which is arranged at the emissive side of the lamp. Such a debris mitigation unit 11 allows the pass of the radiation and suppresses the pass through of the metal vapor. The figure also schematically shows two heater/cooling units 12 for maintaining the metal melt in the baths 2 at a preset temperature.
Such a EUV plasma discharge lamp has the following advantages. Since tin can be used as plasma fuel, a high conversion efficiency of the energy stored on the capacitor bank to EUV is obtained. Since the electrodes rotate, the heat generated by the plasma is spreading over a large surface, which allows high average input powers. The tin layer on the wheels is continuously regenerated, so that electrode erosion does not change the shape of the electrodes. Hence, a very long life time of the lamp is obtained. The liquid tin used for the electrical contact between the capacitor bank and the rotating electrode wheels avoids the requirement of sliding contacts or of a rotating capacitor bank.
The critical region around the plasma is cooled by rotating the electrodes, which means that the input power scales proportionally with the rotation frequency. However, the rotation frequency is limited for the following reason. The centrifugal forces accelerate the tin outwards and at high rotation frequencies droplets are created, i.e. the tin layer tears off. This process can be shifted toward higher rotation frequencies by reducing the thickness of tin film, for instance by appropriate wipers 9. Another possibility is to increase the diameter of the electrodes, which reduces the centrifugal forces (ω2R) at the same velocity (ωR). The drawback of this solution is that extremely large wheels are necessary which improves neither the mechanical stability nor the compactness of the lamp.